1. Field of the Invention
The present invention relates to toner used in an image forming apparatus such as an electrophotographic copying machine or the like, and to a method of manufacturing the same. More particularly, the invention relates to toner capable of retaining excellent electrostatic charge characteristics despite changes in temperature, humidity and other environmental conditions, and to a method of manufacturing the same.
2. Description of the Prior Art
In an image forming apparatus such as an electrophotographic copying machine or the like, a two-component developer consisting of particulate toner and magnetic carrier, or other frictionally chargeable materials, is usually used as the developer to make visible by dry development an electrostatic latent image formed on a photoconductor. A typical method of manufacturing the particulate toner used in the two-component developer is as follows: First, a binding resin which is the main component of the toner, and additives such as colorants for conferring desired properties on the toner are melted and mixed together, thereby uniformly dispersing the toner additives. The mixture is then ground by a mill and classified by a classifier to select toner having a prescribed particle size. Since the particle shape is irregular, the toner manufactured by such method generally has a low flowability and is therefore susceptible to blocking.
Usually, carbon black is added to black toner. When toner containing carbon black is manufactured by the above grinding method, since the obtained toner particles have irregular shapes, carbon black often crops out at the fractured surfaces of the toner particles. This causes the resistance of the toner to be partially reduced since carbon black is a conductor. As a result, the charge amount of the toner charged through friction with the carrier is varied resulting in a variation in image density, fogging of the image, and splashing of toner.
To solve the problem with the above grinding method for the manufacturing toner, there has been proposed a method known as the polymerization method wherein a polymerization reaction is performed with carbon black dispersed in a monomer forming the binding resin. With this polymerization method, however, the carbon black tends to flocculate in the monomer, and toner with carbon black uniformly dispersed in the binding resin cannot be obtained by polymerization alone. If the carbon black is not dispersed uniformly, the electrostatic charge characteristics of the toner tends to wander, causing such problems as splashing of toner and fogging of images.
The present invention, which overcomes the above-described problems, makes possible the objects of providing toner having carbon black uniformly dispersed and therefore having stable electrostatic charge characteristics and be capable of producing high-density images without causing splashing of toner and fogging of images, and also providing a method for the manufacturing the same.